The present invention relates to an imaging lens for forming an image of an object on an imaging element such as a CCD sensor and a CMOS sensor. Particularly, the present invention relates to an imaging lens suitable for mounting on a camera in portable devices such as a cellular phone and a portable information terminal or a relatively small camera such as a digital still camera, a security camera, an onboard camera, and a network camera.
In these years, almost any models of cellular phones have built-in cameras for further added values as the cellular phones. Functions of such cameras have been more sophisticated each year, and especially resolution has been improved significantly. Recently, some of those devices even include built-in cameras with functions that are comparable to those of digital still cameras. On the other hand, so-called smart-phones, which have combined functions of a cellular phone and a portable information terminal (PDA) or a personal computer, are now available and have been purchased more popularly than the cellular phones. Since one of the features of the smart-phones is multifunctional, the smart-phones often include built-in cameras similarly to those of the cellular phones.
An imaging lens for mounting in a camera such as the cellular phone and the smart-phone requires a small size as well as sufficient optical performance that is compatible to the resolution of an imaging element. Conventionally, by using an imaging lens with a two-lens or three-lens configuration, the sufficient optical performance is secured and miniaturization is also attained. However, as the resolution of the imaging element becomes higher, the required optical performance becomes more challenging, and it becomes difficult to sufficiently correct aberrations with the two- or three-lens configuration. Further, it is also difficult to secure the required optical performance.
Moreover, the camera mounted in the cellular phone or the smart-phone is sometimes used for a video call or a self portrait. For example, a person may hold the cellular phone to take a photo with a friend or a photo of the self portrait with a landscape as a background, being a popular way of using the cellular phone especially among young people. In such a case, since the person who takes the photo is an object of the picture to be taken, the imaging lens mounted in the cellular phone requires a larger image-taking range, i.e., a wider angle of view. Furthermore, with improvements of the resolution of the imaging element, there is another common way of enjoying a picture by clipping a desired range from the taken picture. In these uses, the imaging lens is required to have an even wider angle of view and a higher resolution than before.
To this end, a four-lens configuration is considered to be most suitable to solve the above-described problems. As the lens configuration formed of four lenses, for example, the imaging lenses described in Patent Reference 1 and Patent Reference 2 have been developed. Each of the imaging lenses disclosed in Patent References 1 and 2 includes a first lens that is a meniscus lens directing a concave surface thereof to an object side and has negative power; a second lens that is a biconvex lens; a third lens that is a meniscus lens directing a concave surface thereof to the object side and has negative power; and a fourth lens that has positive power.
In the configuration described above, by forming the first lens as the meniscus lens directing the concave surface thereof to the object side, it is possible to attain a wider angle of view of the imaging lens. In addition, by forming the third lens as the meniscus lens directing the concave surface thereof to the object side, it is possible to keep an angle between an incident ray and an emergent ray (angle of deviation) small and restrain generation of aberrations at each refracting surface.    Patent Reference 1: Japanese Patent Application Publication No. 2004-361934    Patent Reference 2: Japanese Patent Application Publication No. 2005-31638
According to the imaging lenses described in Patent References 1 and 2, it is possible to obtain relatively satisfactory aberrations. The functions of the cellular phones and the smart-phones have been more sophisticated each year, and even further miniaturization has been required for the imaging lenses. According to the lens configurations described in Patent References 1 and 2, it is difficult to attain the wide angle while attaining both the miniaturization of the imaging lens and the satisfactory aberration correction so as to meet those demands.
Here, such a challenge is not specific to the imaging lens mounted in the small-sized portable devices such as the cellular phones and the smart-phones, but also a challenge for an imaging lens for mounting in a relatively small camera such as digital still cameras, portable information terminals, security cameras, onboard cameras, and network cameras.
In view of the above-described problems in the conventional techniques, an object of the present invention is to provide an imaging lens that can satisfactorily correct aberrations despite of a small size and have a relatively wide angle.
Further objects and advantages of the invention will be apparent from the following description of the invention.